"Although influenza A viruses have predominated this season, the most recently isolated viruses are influenza B viruses," Jernigan said. This year's vaccine is ineffective against influenza B viruses, he said.
Although the overall effectiveness of this year's vaccine was 44 percent, it has been higher in some years and lower in others, Jernigan said. "In the last 20 seasons, 16 have had good matches, and there have been four that were less than optimal matches," he said.
During the 1997-98 flu season, the vaccine's effectiveness was essentially zero, Jernigan said, adding that was the first year the Type A H3N2 influenza virus appeared.
In some years, the flu vaccine has had an effectiveness level of 70 percent, Jernigan said.
During the current flu season, the number of deaths peaked at 9.1 percent of all reported deaths, Jernigan said. "The number of deaths exceeded the epidemic threshold for 13 consecutive weeks," he said.
There were 66 flu-related deaths of children, 56 of whom weren't vaccinated, were improperly vaccinated or were too young to be vaccinated, Jernigan said.
During the 2003-04 season, deaths peaked at more than 10 percent and remained above the epidemic threshold for nine weeks, Jernigan said.
Each flu season, there are three different strains of influenza virus circulating. Different strains appear at different times, and different strains predominate, Jernigan explained. In addition, each of the three strains can respond differently to the flu vaccine.
It is this fluid flu picture that necessitates changing the vaccine from year to year, Jernigan said.
Since it requires almost a year to prepare the vaccine, decisions about the strains to include in the vaccine are often an educated guess.
But a study published in the April 17 issue of the journal Science could eliminate
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